Speed of Light

Hi David,The speed of light, denoted by c, is a fundamental property of our universe. It just so happens that photons in a vacuum travel at c, so we can measure c by measuring the speed of photons in a vacuum. Photons traveling in a vacuum at speeds other than c would mean our universe is a much different place than the one we live in, and since our universe is precisely the one we do live in we can be sure there are no photons out there bouncing around at supraluminal velocities.If it would satisfy your experimental curiosity you could devise an experiment to measure the speed of light of photons arriving from a distant galaxy.--Percy

You might be reading too much into the phrase "It just so happens," but interestingly, the speed of photons is c in any medium. When traveling through a material like air or glass light travels between molecules at c - the slower effective propagation rate is due to the time taken for absorption followed by emission of a new photon.--Percy